The third-generation semiconductor material GaN has great prospect of application at high temperature and under high voltage due to its high breakdown field strength (greater than 3 MV/cm) and large band gap (3.4 eV at room temperature). Meanwhile, because III-V compound semiconductors have strong spontaneous polarization and piezoelectric polarization effects, two dimensional electron gas (2DEG) channel having a high electron concentration and high electron mobility can be formed in the vicinity of an interface of heterojunctions. Thus, GaN is particularly applicable to high-voltage, high-current, high-temperature, high-speed and high-power devices.
However, in a GaN high electron mobility transistor (HEMT), only 20%-30% of the theoretical voltage withstanding value can be reached, because the electric field will concentrate at an edge of the gate electrode close to the drain electrode when a high voltage is applied to the drain electrode. Therefore, in a GaN HEMT, the breakdown of device usually occurs at an edge of a side of the gate electrode close to the drain electrode.
It is a common method to reduce the electric field peak by using a field plate structure. Generally, an insulating dielectric layer is deposited at the gate electrode first; and then a metal field plate layer is deposited, with the field plate being connected to the gate electrode or the source electrode, and the field plate having a fixed potential. The equipotential of the field plate can adjust power lines at the edge of the gate electrode, so that the gradient of potential becomes flat. In this way, the electric field strength which would have reached the breakdown limit of the material at the edge of the gate electrode is decreased, and the peak is absorbed to the edge of the field plate. This is equivalent to the expansion of the depletion region. The breakdown voltage is much higher since the integral of the electric field is the breakdown voltage. The optimal result of modulating the electric field by using field plates is to allow for an approximately rectangular distribution curve of the electric field. In this way, the integral area of the electric field strength to the distance can be maximized, so that the breakdown voltage of devices can be maximized.
However, the field plate structure will introduce a new electric field peak at the edge close to the drain electrode. Because electric field concentration effect exists at the edge of the field plate electrode as well, the dielectric breakdown at the edge of the field plate is another way resulting in device breakdown. Therefore, during the practical use of the field plate structure, it is necessary to take into consideration the dielectric breakdown of the insulating dielectric in the field plate structure. In addition, the dielectric layer will introduce a capacitance, thereby decreasing the switching speed of the switch in the device and increasing the power loss. Therefore, it is urgent for the skilled in the art to seek for a new field plate technology to increase the breakdown voltage of devices.